The present invention relates to a method and a system for transferring a large amount of data among a plurality of computer-implemented stations or terminals via a network to which the stations or terminals are connected or linked.
The techniques which the present invention concerns are discussed in A. Kamitani: "Outline of Local Area Network-Ethernet", Maruzen Co. ltd, pp. 73, 151-175 and D. Cheriton "REC1045, VERSATILE MESSAGE TRANSACTION PROTOCOL Protocol Specification" (Feb. 1988), pp. 18-21. In these literatures, there are described methods of performing data transactions among a plurality of computers via a network with high reliability by utilizing a connection-oriented service provided in correspondence to a transport layer of an ISO-OSI reference model for establishing a connection between a sender node and a sink or receiver node, whereon communication is performed on the basis of a highly reliable communication protocol which includes a reception acknowledgement and a flow control. On the other hand, in the case where the connection-oriented service involves a remarkably large communication overhead, a connectionless service provided at the transport layer is directly utilized in a higher level layer to thereby realize a highly efficient and reliable data transaction available owing to a combination with the connectionless service.
Further, there has also been proposed a method of decreasing overruns (loss of data packets received by a sink or receiver node) which may occur when the packet transmission rate is high by providing a delay time (inter-packet gap time) between the successive packets for transmission at the sender side and by feeding back or reflecting a retransmission request issued by the sink or receiver station (node) upon detection of occurrence of the overrun to the inter-packet delay or gap time to thereby effect an automatic regulation of the delay time, as is described in D. Cheriton literature.
In general, for realizing a data transmission with a high reliability, it is a common practice to utilize a reliable communication service such as based on TCP/IP (Transmission Control Protocol/Internet Protocol) by establishing a connection between source or sender node and a sink or receiver node. However, in the case where a large amount of data is to be simultaneously multicast or broadcast to a plurality of sink nodes (or receiver terminals), such situation is often encountered that the connection-oriented communications service of high reliability can no more be utilized, making it necessary to develop a reliable processing procedure by combining the connectionless service with the connection-oriented service on the side of the user. In that case, transmission of a reception acknowledgement, sending of a request for retransmission of required data and the like communication are realized by utilizing the connection-less service. Consequently, in order to cope with error which may occur in the above-mentioned communications, the transmission acknowledgement processing, time-out processing and others must be performed in many folds, making it necessary to provide logics for ensuring a high reliability.
On the other hand, on the presumption that the data buffer areas of the sender/receiver nodes or terminals are usually insufficient for the multicast transmission of a large amount of data, it is proposed that the reception acknowledgement is sent back upon every reception of an information frame. In this conjunction, reference may be made to S. Tsuruta and S. Miyamoto: "Proposal and Estimation of Efficient, Reliable and Simple Broadcast Protocol for a Large Amount of Data Transmission", Proceedings of The Information Processing Society of Japan, Vol. 27, No. 4 (April 1986), pp. 462-470. In this literature, it is pointed out that much complicated logics are required for evading the conflict in the reception acknowledgement transmissions from a plurality of receiver terminals in addition to the logic for the aforementioned processings.
Besides, when a large amount of data are transmitted simultaneously at as high a transmission rate as possible, CPU loads of the computers incorporated in the sender/receiver terminals are increased, whereby other processings such as arithmetic operation or the like performed by the computers are subjected to adverse influence, giving rise to a problem in addition to that of increase in the cost of retransmission due to the overrun taking place in the transmission of large amount of data. Among others, in the case of a computer control system which requires a high reliability, it is necessary to maintain the CPU load rate of the computer constantly at a relatively low level to ensure availability of the computer with a margin in the range, for example, of 30 to 40%. Such being the circumstances, the increase in the CPU load due to the transmission of a large amount of data presents a serious problem with regard to the reliability of the computer control system. Additionally, in most of the computer control systems, inter-computer transfer of measurement/control data is realized by resorting to a memory map transmission through periodical short-interval broadcast while omitting acknowledgement of reception. Consequently, transmission or transfer of the measurement/control data may undesirably be obstructed by the transmission of a large amount of data due to limitation imposed on the available buffer capacity. In the hitherto known transmission rate (interpacket time gap) feed-back system operating on the basis of detection of occurrence of overrun, it is contemplated to optimize the transmission cost involved only in the transmission of the sender terminal, and no consideration is paid to the securement of overall reliability for the communications of the whole system inclusive of other communications and operations in addition to the transmission of large amount of data by preventing the last mentioned data transmission for a given terminal from exerting influence to other communications for that terminal or station.